The invention relates to a spreading-wedge device, particularly for motor-vehicle brakes, and more specifically to such a device which includes a housing attached to the brake carrier, two. coaxial pressure means which are displaceably mounted in the housing and abut against a brake-operating member, a spreading wedge which engages against a plurality of rollers which in turn engage against the two pressure means, the wedge extending to the outside through an opening in the housing neck, including a spreading-wedge shaft, a diaphragm cylinder or similar item, serving the loading of both the pressure means and thus the brake, which axially abuts against the spreading-wedge shaft, and a sealing unit, arranged between the spreading-wedge shaft and the housing neck, with a flexible sealing cover which extends, via a diversion area between two ring-shaped edge rims of different diameters, from where the edge rim with smaller diameter abuts against the spreading-wedge shaft and the edge rim with larger diameter is fixed within the area of the housing neck, and with a return spring embracing the spreading-wedge-spreading-wedge shaft unit.
Such spreading-wedge devices with flexible sealing cover are to satisfy the requirement that no dust- and dirt particles or dampness can enter via the diaphragm cylinder into the greasefilled housing interior and mix with the grease which can cause corrosion and increased wear at the functional parts and hence premature failure of the spreading-wedge device.
In an already known spreading-wedge device of the initially described type European Patent Specification, corresponding to U.S. Pat. No. 4,519,482, the smaller edge rim has been placed for this reason at the spreading-wedge edge between a spring bushing and a pressure-spring loaded disc. A further spring, serving as a return spring, is provided in the form of a pulling spring which abuts against the housing in the area of the fixing location of the larger edge rim. The sealing cover embraces an approximately cylindrical section with larger diameter, adjacent to which is a cylindrical section with smaller diameter. It has been found that, with activation of the spreading wedge via the diaphragm cylinder axially into the housing, disadvantageously due to the cylindrical arrangement of the two cover sections, no roll-in movement takes place. Instead, folds develop in the connection area of the two cover sections, leading to the development of cracks. Since such a sealing device is required to withstand several hundreds of thousands of brake-operating lifts, a premature development of cracks causes failure of the spreading-wedge device.
It has been found that fixing the sealing cover via the two springs, which are required for its support or the spreading-wedge return respectively, and thus squeezing on the rims also results in the formation of cracks and hencewith in failure of the sealing cover.
It has been tried to counter these disadvantages European Patent Specifications Nos. 106,954 and 261,903 by constructing the sealing cover to be inflexible so that it cannot follow the axial path of the spreading wedge but instead acts as a sliding device via its inner circumference abutting against the spreading-wedge shaft. However, the following disadvantages were discovered: due to the friction, a loss in the degree of effectiveness occurs. Furthermore, a grinding effect between the spreading-wedge shaft and the inside circumference of the sliding device causes increased wear of those parts. Finally, it has been found that during operation of the brake, foreign particles accumulating at the spreading-wedge shaft press against the seal and can penetrate into the inside of the housing. In addition, in the embodiment according to European Patent Specification No. 106,954 cracks may form in the exterior rim region due to squeezing by the pressure spring, or intrusion inbetween the spring windings if folds develop at the outer circumference of the seal. Since the larger rim is firmly fixed by screwing in the guide tube of the diaphragm cylinder, grease contained in the inside area of the spreading-wedge device can exit while being mounted or being in a state of transport because of loose mounting of the diaphragm cylinder inside the housing neck due to lack of seals. Conversely, dirt and wet particles can enter the inside of the spreading-wedge device.